Green’s Dictionary of Slang

1. a long, deep glass of beer.

1909		Bulletin (Sydney) 17 Feb. 13/2: I have discovered the longest long beer in the Commonwealth, longer than the ‘long sleever’, deeper than the ‘Dreadnought’, and bulkier than the beers of Bourke.
1934		Bulletin (Sydney) 21 Feb. 10/2: This is a thin glass holding a quart which is affectionately referred to by its intimates as a ‘dreadnought’.
1939		K. Tennant Foveaux 25: Behind them came a little perambulator with a notice reading ‘Australia’s Dreadnought’ which had an uproarious reception.

2. the leading shearer in a shed, dealing with 200+ sheep in a day; also attrib.

1910		Morn. Bulletin (Rockhampton, Qld) 15 Aug. 4/6: It will not be long before the words ‘ringer’ and ‘gun’ as applied to crack shearers in the west will be obsolete. A new term is already being used [...] ‘Dreadnought’.
1924		Morn. Bulletin (Rockhampton, Qld) 19 July n.p.: The sheep there were tough [...] I know that the ‘gun’ or the ‘dreadnought’ did not nearly reach the second century. Yet the same men could go over to Wellshot [...] and get well over 200.
1931		Charleville Times 6 Nov. n.p.: Shearing Tallies ‘The Dreadnought Class’ Some past performances.
1982		Sydney Morning Herald 23 Oct. 29/4: The mighty men who could knock up a tally of 300 [sheep] a day are dreadnoughts.